This invention relates generally to moment balancing mechanisms and, more particularly, to a linkage for use in moment balancing measuring apparatus of the pneumatic type including a linkage comprising first and second levers, the linkage being adapted to be acted upon by a measuring force derived from a measuring element and wherein the linkage is balanced by a pneumatic force applied thereto.
Pneumatic moment balancing apparatus are known wherein a measured value in the form of a force is converted to a corresponding air pressure for transmission of the measured value over a distance.
More particularly, in conventional pneumatic balancing apparatus, the measuring force f.sub.m generated by a measuring element acts on a beam mounted for rotation in a plane with respect to a pivot point. The beam is maintained in a balanced condition by a pneumatic force f.sub.p which results from air pressure acting within a corrugated or bellows type tube. Compressed air is conducted into a pneumatic system through a throttle and a flap which is affixed to the balance beam and regulates the air flow discharging through a nozzle in accordance with the position of the beam thereby resulting in a particular pressure arising within the pneumatic system.
The pneumatic system of such conventional moment balancing apparatus is designed such that only a minimal movement of the flap, e.g., about 0.1 mm, will cause the air pressure within the pneumatic system to vary through the entire standardized signal range, e.g., 0.2 to 1.0 bar.
Therefore, as the measuring force f.sub.m varies, the air pressure within the pneumatic system varies accordingly and the leverage ratio, f.sub.p /f.sub.m is automatically maintained constant.
The pneumatic system is provided with an output connector from which air pressure proportional to the measuring force may be transmitted into a conduit through which the same is transmitted so as to constitute a measuring signal at a remote location.
It is desirable to provide such moment balancing apparatus with a capability such that the ratio f.sub.p /f.sub.m can be varied in order to adapt the apparatus to the particular range of variation of the measuring force f.sub.m for any particular instance of measurement. It is advantageous to provide a relatively wide range of variability for the leverage ratio f.sub.p /f.sub.m so that the size of the measuring elements, e.g., the diaphragm or corrugated tube elements which convert a pressure to a measuring force, can be maintained relatively small. Thus, attempts have been made to provide linkages wherein the maximum lever ratio f.sub.p /f.sub.m is about 10 to 15 times its smallest usable value. The variation of the lever ratio f.sub.p /f.sub.m has been accomplished in prior art apparatus through appropriate movement of the fulcrum point of the balance beam.
The arrangement described above for varying the lever ratio is not entirely satisfactory for several reasons. Firstly, it is difficult using such an arrangement to achieve a relatively wide range of variation for the lever ratio f.sub.p /f.sub.m. Secondly, only a single definite position of the fulcrum will result in the beam being in a precise balanced position. In other words, when the beam fulcrum is displaced in order to vary the ratio f.sub.p /f.sub.m, an imbalance occurs which results in the apparatus becoming particularly sensitive to vibrations. Thirdly, the displacement of the fulcrum of the balance beam is an awkward operation since the fulcrum must be dismounted and then remounted at both of its ends and, further, during displacement care must be taken to move both ends through the same distance.
Another type of apparatus wherein the lever ratio f.sub.p /f.sub.m can be varied has a so-called double lever design. The balancing of a device of this type is hampered, however, by the movable lever connecting piece. Additionally, the displacement thereof is a relatively cumbersome operation.
Yet another prior art linkage construction is known wherein the particular leverage ratio f.sub.p /f.sub.m is set by adjusting the angle .alpha. between two levers. Although the linkage remains balanced in this apparatus, it is difficult in practice to obtain the desired adjustment range for the leverage ratio for the reason that with small angles .alpha. the temperature of the apparatus exerts an increased influence on the measuring signal so that the setting of the leverage ratio f.sub.p /f.sub.m accordingly becomes difficult since the sensitivity to small changes in such adjustment is quite high. Reference is made to U.S. Pat. No. 3,739,655 in which apparatus of the type described above is disclosed.